What can K2 tell us about the TRAPPIST1 planetary system?

What can K2 tell us about the TRAPPIST1 planetary system?

The TRAPPIST1 planetary system was announced two weeks ago to the excitement of many. TRAPPIST1 is an ultracool dwarf star that hosts at least seven planets that are all roughly Earth-sized. There are so many remarkable things about this system, but arguably the most important feature is that the discovery team was able to measure masses. With both sizes and masses constrained, the TRAPPIST1 planetary system will enable a wide range of characterization studies and predictions that will be testable with upcoming surveys and space missions.

NASA’s K2 Mission observed TRAPPIST1 during an ~80 day campaign from December 15, 2016 – March 4, 2017 at short (1 minute) cadence. On March 8 the raw data will be released and made publicly available on the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute in Maryland.  Because the data will be in a raw format (normally this type of data would be released weeks or months after internal teams have had time to process it) members of the K2 Mission Guest Observers Office and the MAST team are preparing to help support users. On the MAST website (http://archive.stsci.edu/k2/trappist1/) you can access the data and in the comments section (shown below, Scott Fleming represents the MAST support and Geert Barentsen is from the K2 Mission) users are encouraged to share their tools and results. It should be a fun day!



On the 8th anniversary of Kepler’s launch (3/7/09), it seems fitting to have a major development take place in the rapidly growing field of exoplanets. The beauty of the K2 Mission (Kepler’s extended mission) is that it is 100% community-driven and all data is open to the public with no proprietary period. That means that a high school student can participate in the glorious digging of spacecraft data alongside professional astronomers.

What do we expect to find? Let’s first take a brief look at the development of the TRAPPIST1 planet discoveries over the past year and what we know about them so far.

Three planets orbiting TRAPPIST1 were first announced in May 2016 in an article titled ‘Temperate Earth-sized planets transiting a nearby ultracool dwarf star‘.   The planets were discovered by a Belgian-led team using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at the La Silla Observatory in Chile. At the time I was awestruck, primarily because I’m a nerd but also because of the size of the star. TRAPPIST1 is a class M8 star that is about 40 light years away (12 parsec) in the constellation Aquarius and has a mass of 0.08 times the mass of our Sun. M class stars (or M dwarfs, red dwarfs) span a mass range of about 0.08 to 0.5 times the mass of our Sun. TRAPPIST1 is essentially the smallest a star can be, any less massive and it wouldn’t be able to fuse hydrogen into helium in its core (i.e., what stars do). Kepler has revealed lots of multiplanet systems around M dwarfs, including around Kepler-186f which is 0.5 times the mass of the Sun, but none as small as TRAPPIST1. With the announcement of the TRAPPIST1 planets, it became evident that Earth-size planets form around stars that span the full range of M dwarf masses! This is a big deal because more than 70% of stars in the solar neighborhood are M dwarfs, so knowing that other Earth’s can form around all types of M dwarfs has big implications for the abundance of potentially habitable planets in our galaxy. While this system was exciting when announced, these planets did not yet have their masses measured.

Cut to February 2017, and after the TRAPPIST1 team collected more data, including Hubble and 20 days of Spitzer data, they were able to announce that their full set of data has revealed the presence of 7 ~Earth-sized and ~Earth-mass planets. To date, there are only three other 7 planet systems, two are systems found by the radial velocity technique (HD 10180 and HR 8832) and the third is Kepler-90, a G-type star with 5 inner planets with sizes between 1.3 and 2.9 Earth radii and 2 outer gas giant planets. Only our Solar System currently has > 7 planets.

This time, the team had enough data coverage to measure Transit Timing Variations (TTVs), which are variations in the times that the planets transit (from strictly periodic) induced by tugs from neighboring planets. Eric Agol, who participated in the discovery, was in fact among the first to write about the TTV method in 2004!

The orbital dynamics of the system are incredibly rich. From the discovery paper, the inner six planets (b, c, d, e, f, g) have orbital periods with “ratios Pc/Pb, Pd/Pc, Pe/Pd, Pf/Pe, and Pg/Pf close to the ratios of small integers 8:5, 5:3, 3:2, 3:2, and 4:3, respectively”. For example, the innermost planet b will orbit 8 times while the next planet c will orbit 5 times in the same duration, etc. Orbital resonances such as this are suggestive of a formation scenario that includes some form of inward migration.

What likely received the most attention is that three planets orbit within the stellar habitable zone (HZ), a region that (given an Earth-like atmosphere) a planet can have liquid water on its surface. A team working on Kepler planet occurrence rates once asked me to look into how many Earths you could pack into a HZ, and we did find you could often fit three planets – but until TRAPPIST1 there hadn’t been any such cases. What is most interesting in my opinion is that three of the planets receive stellar flux that is comparable to what Venus, Earth and Mars receive from the Sun. If we want to study why Earth has an atmosphere that allows life to thrive, but not Venus (which is close in size and density to Earth), systems like TRAPPIST1 will be valuable targets for future space telescopes like the James Webb Space Telescope.

I’ll refrain for now from talking about the potential habitability of these planets (answer: “It’s Complicated”) I’ll note that yet another feature of stars with masses this low is their longevity, they can live for trillions (trillions!) of years. So planets in their HZs can remain in their HZs for a really long time.

Another remarkable feature of TRAPPIST1 is that it resembles the Jupiter-Galilean satellite system more than it resembles our Solar System, as the size of the star is just a little bigger than Jupiter, and the ratio of the planet masses to the star mass is comparable to the ratio of moon masses to their giant planet host. The formation mechanisms to form these planets might be closer to giant planet formation theories than to Solar System formation theories. Now that we know their masses, we can more easily justify and refine planet formation models.

So back to K2 – and what do we expect to find tomorrow?

K2 will have collected data for about 80 days, 4 times longer than what was collected with Spitzer. If you take a close look at the table of planet parameters for the seven planets below, you’ll notice that the error bars on some of the planet masses are quite big. K2 data may help tighten these (which will help with all studies of these planets). You will also notice that only 1 transit was collected for the 7th planet “h”. There is therefore a larger uncertainty to the orbital period of planet h. Nominally, it is 20 days, but it could be as short as 14 days and as long as 35 days. If it happens to have a shorter orbital period, then there is a good chance K2 can reveal additional planets if they exist. If it turns out that planet h has, say, a 35 day period, then we may get up to two transits with K2 which can help constrain its own properties. If it turns out there is a big gap between planet g (12.4 days) and planet h, then the next logical question would be — can there be more planets in between? With Kepler-186f we found that you could have a stable planet in between planets e and f, it would just have to be an Earth-mass or less and inclined by just a few degrees for us to not see it transit. However, the TRAPPIST1 is a much more compact system and will probably be full of surprises.


Edit: Tom Barclay from the K2 Mission has developed an iPython notebook here to create light curves from the raw TRAPPIST1 data and look for planets. This and other updates are posted on the MAST’s TRAPPIST1 page.

More information on K2 and its sensitivity to ultracool dwarfs like TRAPPIST1 can be found in this article Probing TRAPPIST-1-like Systems with K2 by Brice-Olivier Demory, who is on the TRAPPIST1 discovery team and has been working on searching for planets around ultracool dwarfs for some time. The latest article TRAPPIST1 is here: “Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1“.


Watch these videos to hear some brilliant astrophysicists talk more about the TRAPPIST1 system!